The "recognition" between macromolecules play a fundamental role in most of biological processes in the living cells; recognition between plus strand and minus strand of double stranded DNA and RNA, between DNA and histones, between operator and repressor, between tRNA and its cognate aminoacyl synthetase, between DNA (or RNA) and certain mutagens or dyes, and enzyme and its substrate, etc. To understand the mode of recognition, one has to know the tertiary structure of the molecular complex involved at the atomic level. Our objectives are: (1) to obtain single crystals of the molecular complexes between RNA and various interacting molecules, such as aromatic mutagens, aromatic carcinogens, dye molecules, antibiotics, basic peptides, complimentary oligonucleotides (ribose and deoxyribose forms); (2) to determine the tertiary structures of these molecular complexes using the X-ray crystallographic method; (3) to consider these molecular complexes as model systems and interpret the results, at the atomic level, in light of nucleic acid-aromatic mutagen (or carcinogen) interaction, DNA-histone interaction, DNA-RNA interaction, RNA-RNA interaction, codon-anticodon interaction.